In FIG. 1A, there is shown a conventional laminated insulation paper 30, used as a low-loss insulation paper for an electric power cable, which paper is integrally formed of two cellulose papers 10 and 12 and a film-like plastic sheet 20 sandwiched therebetween.
In the cable insulation assembly using the laminated insulation paper 30 thus constructed, oil flow resistance in the radial direction is undesirably very large.
Under this circumstance, it has been proposed to provide through holes 22 in the plastic sheet 20 as shown in FIG. 1B. An example of such a construction is disclosed in British Patent 1,057,744. In the cable insulation assembly using a tape with the above through holes thus constructed, the through holes 22 undesirably form oil gaps, where the gap length d in the electric field direction is equal to the thickness of the plastic sheet 20, to thereby cause an electrical defect.
Further, the conventional laminated insulation paper 30 having the plastic sheet 20 with the through holes 22 has been made in a manner such that, at first, through holes 22 are provided in the plastic sheet 20 as shown in FIG. 1(C); cellulose papers 10 and 12 are then laminated on the front and back surfaces of the plastic sheet 20 respectively, and are then passed through a pair of heat rollers 40 and 42 to thereby integrally form the three layers.
In such a case, however, the plastic sheet 20 is heated by the heat rollers up to near its melting point and is pressed by the heat rollers, and therefore the through holes 22 have been apt to be crushed.